For the MkIV: The headliner consists of three primary pieces: the backing, the foam liner attached to the fabric, and the fabric itself. The fabric and foam are attached to the backing by glue, but the glue itself often is not the failure, it's the disintegration of the foam, which allows the fabric to droop. The fabric gets peeled away, the remnants of the foam removed, and the backing cleaned up to receive new fabric/fabric+foam. The headliner backing is (IMO) not like door card inserts, and will bend and crease under its own weight, and can tear if caught on anything when being lowered or removed. Any heavy rubberized spray coating, separate from concerns over "inside" use, could easily weigh down the headliner backing. The vortex does have lots of creative solutions, and there's a whole thread of headliner projects over there, but I wouldn't pick bedliner as a top choice.